Photon-stimulated desorption and the effect of cracking of condensed molecules in a cryogenic vacuum system

The design of the large hadron collider (LHC) vacuum system requires a comp lete understanding of all processes which may affect the residual gas densi ty in the cold bore of the 1.9 K cryomagnets. A wealth of data has been obt ained which may be used to predict the residual gas density inside a cold v acuum system exposed to synchrotron radiation. In this study, the effect of cracking of cryosorbed molecules by synchrotron radiation photons has been included. Cracking of the molecular species CO2 and CH4 has been observed in recent studies and these findings have been incorporated in a more detai led dynamic gas density model for the LHC. In this paper, we describe the r elevant physical processes and the parameters required for a full evaluatio n. It is shown that the dominant gas species in the LHC vacuum system with its beam screen are H-2 and CO. The important result of this study is that, while the surface coverage of cryosorbed CH4 and CO2 molecules is limited due to cracking, the coverage of H-2 and CO molecules may increase steadily during the long-term operation of the machine. (C) 2001 Elsevier Science L td. All rights reserved.